1. Field of the Invention
The present invention relates to a strip-line filter for use in a microwave band and an extremely high frequency band, a duplexer, a filter device, a communication device, each including the same, and a method of a characteristic of the strip-line filter.
2. Description of the Related Art
Conventional strip-line filters are disclosed in Japanese Unexamined Patent Application Publication No. 56-116302, U.S. Pat. No. 3,451,015, and Japanese Examined Patent Application Publication No. 62-19081 (U.S. Pat. No. 4,352,076).
In Japanese Unexamined Patent Application Publication No. 56-116302, plural resonator electrodes each constituting half-wave resonators are arranged substantially in parallel to each other on a substrate, and lead-out electrodes are connected to the resonator electrodes of the first and last stages.
U.S. Pat. No. 3,451,015 discloses a strip-line filter in which plural resonator electrodes each constituting half-wave resonators or quarter-wave resonators are arranged substantially in parallel to each other on a substrate, and lead-out electrodes are connected to the resonator electrodes of the first and last stages.
In Japanese Examined Patent Application Publication No. 62-19081 (U.S. Pat. No. 4,352,076), a strip-line filter is disclosed in which plural resonator electrodes each constituting a half-wave resonator are arranged substantially in parallel to each other on a substrate, and a coupling conductor forms a static capacitance with the resonator electrodes for coupling the resonator electrodes with an opposite phase so that an attenuation pole is developed.
In the case of a strip-line filter in which an attenuation pole is developed by coupling the resonator electrodes with opposite phase, as described in the above-mentioned Japanese Examined Patent Application Publication No. 62-19081, the band-pass filter can be provided with a steep attenuation characteristic in the range between the transmission band and the attenuation band.
Japanese Unexamined Patent Application Publication No. 56-116302 and U.S. Pat. No. 3,451,015 do not describe such strip-line filters having such attenuation poles developed therein.
A disadvantage of the above strip-line filter, having coupling with opposite phase between the input and output stages through a static capacitance, is that the transmission characteristic of the pass band is unnecessarily reduced, since attenuation poles are produced on both the higher and lower sides of the pass-band. That is, the insertion loss generated in the pass band may be increased, or the pass band width may become too narrow.
Furthermore, the static capacitances between the electrode patterns are somewhat unpredictable, due to variations in the sizes of the electrode patterns. This causes the problem that stable attenuation poles can be obtained with difficulty.
Accordingly, the present invention provides a strip-line filter in which a stable attenuation pole is generated on one side, that is, on the lower or higher side of the pass-band, without the input and output being coupled by means of a static capacitance. Thus, the above-described problems are solved.
The invention further provides a duplexer, a filter device, a communication device including the filter, and a method of adjusting the filter characteristic of the strip-line filter.
To provide these advantages, a first aspect of the present invention provides a strip-line filter which comprises plural resonator electrodes each constituting a half-wave resonator arranged in one direction on or inside of a substrate, and lead-out electrodes connected to the resonator electrodes of the first and last stages, at least one of the resonator electrodes of the first and last stages having a ratio (W/L) of an electrode width W to an electrode length L in the range of about 1.05 less than W/L less than 1.95, in which the electrode length L is an electrode length of the resonator electrode measured perpendicular to the direction in which the resonator electrodes are arranged, and the electrode width W is an electrode width of the resonator electrode measured parallel to the arrangement direction. Further, the lead-out electrodes are connected to the resonator electrodes of the first and last stages on opposite sides of the center axis, which is a straight line axis passing in said arrangement direction through the center positions along said length direction of the resonator electrodes of the first and last stages.
As seen in the concrete examples, namely, the embodiments described below, experiments by the inventors have revealed that the above-described configuration causes an attenuation pole to develop on the lower side of the pass-band. In the present invention, the attenuation characteristic is steeply changed in the range from the pass-band to the attenuation band on the lower side. Furthermore, no attenuation pole is generated on the higher band side of the pass-band, and the transmission characteristic in the pass-band is not deteriorated.
Furthermore, according to a second aspect of the present invention, there is provided another strip-line filter which comprises plural resonator electrodes each constituting half-wave resonators arranged in one direction on or inside of a substrate, and lead-out electrodes connected to the resonator electrodes of the first and last stages, at least one of the resonator electrodes of the first and last stages having a ratio (W/L) of an electrode width W to an electrode length L in the range of about 0.10 less than W/L less than 0.95. Further, lead-out electrodes are connected to the resonator electrodes of the first and last stages on the same side of the center axis.
As seen in the concrete examples, namely, the disclosed embodiments, experiments by the inventors have revealed that the above-described configuration causes an attenuation pole to develop on the higher side of the pass-band. In the present invention, the attenuation characteristic changes steeply in the range from the pass-band to the attenuation band on the higher side. Furthermore, no attenuation poles are generated on the lower side of the pass-band, and the transmission characteristic in the pass-band is not deteriorated.
Preferably, the lead-out electrodes each are led-out from the strip-line filter substantially at the ends of the center axis, and function as input-output terminals. Thereby, the substrate having the filter configured thereon and electrodes provided on a circuit board or package for mounting the substrate can be connected more effectively.
A duplexer in accordance with the present invention comprises two of the above-described strip-line filters. Thereby, a duplexer with increased attenuation in a required frequency band is provided.
Preferably, the duplexer comprises one strip-line filter of one of the above two types and one strip-line filter of the other type. Thereby, in the case in which one filter constitutes a transmission filter, and the other filter constitutes a reception filter, the attenuation characteristic changes steeply at the boundary between the adjacent transmission and reception bands, so as to suppress leakage of a transmission signal to the reception circuit.
A filter device in accordance with the present invention is formed by mounting the above-described strip-line filter or duplexer to a cover, a casing, or a waveguide having a cut-off frequency which exerts no influence over the filter characteristic.
Furthermore, in a communication device in accordance with the present invention, the above-described strip-line filter or duplexer is disposed, e.g., in a filter section or an antenna sharing device section for carrying a transmission or reception signal in a high frequency circuit.
According to the present invention, there is provided a method of adjusting the filter characteristic of a strip-line filter which comprises the steps of providing a frequency adjustment electrode protruding from at least one of the resonator electrodes, preferably perpendicularly to the arrangement direction of the resonator electrodes in the above-described strip-line filter, and removing a predetermined amount of the frequency adjustment electrode to adjust the center frequency of the filter.
Moreover, there is provided another method of adjusting the characteristic of a strip-line filter which comprises the step of providing an external coupling adjustment electrode protruding from at least one of the lead-out electrodes, preferably perpendicularly to the arrangement direction of the resonator electrodes, and removing a predetermined amount of the external coupling adjustment electrode to adjust the external coupling of the filter.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.